In the information storage industry, increases in the data capacity of tape have been achieved with thinner tape substrates and with various data compression techniques. Advances in the magnetic tape media and tape head technologies have generated further increases in data capacity by increasing both the number of data tracks on a magnetic tape (by narrowing the width of each track) and the number of data read/write elements on the head.
The number of parallel, longitudinal data tracks which can be established on tape media has been limited by the number of read/write elements which could be fabricated on a head to read/write narrower tracks. Therefore, data tape drives have been designed to process a tape using a head having fewer sets of read/write elements than there are tracks on the tape. The tracks are divided into groups, each containing the same number of tracks as there are read/write elements in the head. To access all of the groups the head is indexed transversely relative to the tape width into a number of discrete positions corresponding to the number of groups of tracks. For example, a head having eight read/write elements can accommodate a tape having twenty four tracks if the tracks are divided into three groups of eight tracks each and the head has three index positions.
However, even when a head is indexed, there is a practical limit to the ability of a multi-track head to accurately and reliably record data to and read data from a tape having such a large number of very narrow tracks. Problems can be caused by track misregistrations, such as tape edge variations, environmental thermal expansion and contractions, inaccuracies in the path the tape follows in a drive, inaccuracies in the formatting of tracks on the tape itself, and dimensional and spacing deviations during the manufacturing of the head. It can be appreciated that even a minute "wobble" in the tape or a misalignment in the head can result in significant signal degradation, such as crosstalk and dropout, if a 12.7 mm tape has 128 tracks, each with a width of about 80 microns.
Consequently, a tape head actuator has been developed to index a tape head to one of several positions during track seek operations. For example, to access a tape having 128 tracks, a head having thirty two read/write elements indexes among four positions. Moreover, the actuator is also capable of rapidly adjusting the position of the head under servo control to precisely follow a set of tracks during read and write operations. In a drive employing such an actuator, the tape head has servo read elements for reading servo signals previously recorded onto one or more specially recorded servo tracks. Each servo element generates a position error signal which a position servo loop employs to determine the transverse position of the servo elements relative to the servo tracks. The loop then transmits a signal to the actuator to rapidly move the head by very small amounts as necessary to enable precise track following.
To improve the accuracy with which the servo loop operates, the tape can have two or more servo areas, each including a set of one or more servo tracks, spaced across the width of the tape. The tape head has a corresponding number of sets of servo elements. The position error signals generated by the servo elements are concurrently read and averaged to obtain a single feedback signal. The head position is maintained by the servo loop in response to the feedback signal, rather than the position error signal from any one servo read element. Such redundancy makes the servo loop less susceptible to error or failure by any one servo element. One such system includes three symmetrically spaced servo areas, each having three adjacent servo tracks, parallel to the data tracks.
However, accuracy problems in the position error signals can be caused by various sensor data integrity problems such as noise, servo format recording problems, media defects, tape to head interface variations, and servo channel hardware problems. Accordingly, a problem with averaging all of the position error signals to obtain the feedback signal is that the integrity of the feedback signal may not represent the true head position relative to the servo track if at least one of the position error signals is highly inaccurate.